1. Field of the Invention
The invention relates to a square-law detector having an input, an output and means for providing a square-law relationship between the amplitude of the signal applied to the input and the amplitude of the signal produced on the output.
Such a square-law detector is suited for use in a receiver for polarization diversity reception with square-law recombination of baseband signals, such as described in EP-A-474.294.
2. Description of the Related Art
For transporting a baseband signal through a glass fibre in coherent optical transmission systems the light signal from a transmitting laser is amplitude, frequency or phase modulated by the baseband signal before the light signal is fed to the glass fibre.
For demodulating the light signals at the receiver end by conventional electronic components, it is necessary to convert a light signal which has a very high frequency (for example, 10.sup.14 Hz) to a much lower intermediate frequency (for example, 10.sup.9 Hz). For this purpose, the received light signal is mixed in the receiver with a light signal locally generated by a laser, which mixing operation is performed by an optical directional coupler. As a result, an intermediate frequency signal is obtained having a frequency equal to the difference frequency between the frequency of the received light signal and the frequency of the locally generated light signal.
To have the least possible signal loss due to this mixing operation, it is necessary for the direction of polarization the received light signal and the direction of polarization of the locally generated light signal to be the same. In general the direction of polarization of the received light signal, however, is indefinite and, in addition, not constant in time. Without any precautions being taken, the amplitude of the intermediate frequency signal may therefore vary between a maximum value (if the two directions of polarization are the same) and substantially zero, (if the two directions of polarization are orthogonal).
In general, the problem is solved by splitting the received light signal into two components that have mutually orthogonal directions of polarization. Each signal component is separately mixed with a corresponding polarized component of the locally generated light signal. This mixing produces two intermediate frequency signals. After amplification and demodulation of the intermediate frequency signals, two baseband signals are available whose amplitudes are proportional to the amplitudes of the two mutually orthogonally polarized components of the received light signal.
For obtaining a baseband signal whose amplitude is proportional to the amplitude of the received light signal and irrespective of the direction of polarization thereof, the two baseband signals can be squared and then added together according to their known vector properties. In lieu of squaring the obtained baseband signals, the desired output signal may also be obtained by utilizing a demodulator that has a square-law relationship between the amplitude of the input signal and the amplitude of the output signal.
Another application of a square-law detector is described in GB-A-90.27296. This publication also discusses a receiver for use in a coherent optical transmission system and, more specifically, discusses an AGC system for such a receiver. In this AGC system a discriminator circuit and an intermediate frequency power detector are used. These circuits both include a detector circuit having a square-law characteristic.
If FSK (Frequency Shift Keying) modulation having a large frequency deviation (.DELTA.f) compared with the bit rate is used in an optically coherent transmission system, the modulated signal spectrum has two relatively discrete power peaks as is shown in FIG. 1a. If such a spectrum passes a detector which has a substantially rectangular characteristic and a bandwidth B, as is the case in known detectors and is shown in FIG. 1b, the output signal of the detector plotted against frequency will have the step size shown in FIG. 1c. Such a step size with discrete jumps in the voltage levels is undesired especially in control loops because instabilities will then arise as a result.